1. Technical Field
This invention relates to the bonding of articles to electronic component packages. More specifically, the invention relates to a method for bonding heat sinks to overmolds and a device formed thereby.
2. Background Art
Electronic components, such as semi-conductor devices, are used in increasing numbers in a wide variety of products. Generally, electronic components produce heat during operation. In some devices, the heat generated by an electronic component may build in the component, or in the device, and damage the component or other components in the device. Accordingly, there is often a need to dissipate the heat generated by electronic components to extend the life of devices using such components.
Several ways of dissipating heat generated from electronic components have been developed. One common method is to provide a fan in the device to blow air through the device and to vent the heat. Unfortunately, a fan cannot always be provided in a device using electronic components. Another way to increase heat dissipation from an electronic device involves increasing the surface area of the device. According to well recognized heat transfer principles, increasing the surface area will cause an increased transfer of heat from the electronic component to its surroundings. Unfortunately, it is generally desirable that electronic components be as small as possible and there is an increasing need to reduce the size of components while still providing sufficient heat dissipation. Yet another way to dissipate heat is to bond an electronic component to a heat sink. The purpose of a heat sink is to conduct heat away from the electronic device and then dissipate the heat from the heat sink. Heat sinks are typically made from a heat conductive material, such as metal, and aluminum is frequently used since it is light weight and readily available. Such heat sinks can easily be designed to conduct heat away from the electrical component while simultaneously providing a large surface area for heat dissipation. For example, the heat sink may include integral fins that provide a large surface area for a relatively small volume.
While heat sinks solve some of the problems of heat dissipation, there remains a current problem of bonding a heat sink to an electronic component. For the heat generated by the component to be transferred to the heat sink, a connection allowing heat conductance between the component and the heat sink must exist. Conventionally, heat sinks are mechanically attached to electronic components and a heat conducting thermal grease is placed between the component and the heat sink, thus, providing heat conductance as needed. Unfortunately, mechanical attachment with clips, rivets, or other devices possesses serious disadvantages. First, such mechanical attachments require thermal grease, clips, and/or rivets, and other materials, increasing the material cost of a unit with a heat sink. Second, the mechanical attachments increase the process costs. Third, such mechanical attachments have proven to possess poor long term reliability. In other words, either the conductive path between the component and heat sink is compromised or the mechanical attachment fails all together. In addition, devices such as rivets put stress on the electronic component and may cause failure of the component during thermal expansion and contraction.
One attempt at resolving the problem of mechanical attachment involves using adhesive to create a uniform bond between an electronic component and a heat sink. Unfortunately, many electronic components are made of substances or are packaged in substances to which it is very difficult to adhere a heat sink. For example, electronic components are often at least partially encapsulated in polymer compounds through injection molding or other molding processes. Typical adhesives that are expected to bond with polymer compounds will not bond a heat sink to some encapsulants. This problem has been encountered in the production of plastic ball grid array (PBGA) packages, in particular, PBGA packages with an overmold covering electronic components mounted on the PBGA.
Thus, there existed a need to provide a method for uniformly bonding heat sinks to electronic component encapsulants.
According to the present invention, a method for bonding is provided comprising the steps of exposing to a plasma a surface of a molded polymer formed on a substrate, allowing the plasma to at least partially convert silicon-containing residue on the surface to silica, and bonding an article to the surface by applying an adherent between the article and the surface. By way of example, the plasma may be an oxygen plasma. Also, the molded polymer may be an overmold, the substrate may be an electronic component, and the article may be a heat sink. Further, the step of bonding, for example, may include heat curing the adherent by preferentially driving heat through the article to avoid exposing the substrate to the curing temperature. One example of a suitable adherent is a silicone-based paste adhesive with a metal oxide filler.
The present invention provides another method for bonding comprising the steps of providing a molded polymer formed on a substrate, wherein the molded polymer has a surface with a silicon-containing residue thereon, bonding an article to the surface by applying a porous polymer film between the article and the surface, wherein the film is impregnated with adhesive, and heat curing the film. By way of example, the silicon-containing residue may be silicone oil, the film may be polytetrafluoroethylene, the adhesive may be polybutadine, and the film may be further impregnated with a metal oxide heat transfer medium.
The present invention also provides an apparatus comprising a molded polymer formed on substrate, a silica layer on a surface on the molded polymer, adherent bonded to the silica layer, and an article bonded to the adherent. The silica layer thus provides a surface on the molded polymer to which an adherent may be adequately bonded. Another apparatus comprises a molded polymer formed on a substrate, silicon-containing residue on a surface of the molded polymer, an adherent bonded to the surface, and an article bonded to the adherent. By way of example, the adherent may be a porous polymer film impregnated with the adhesive as described above. In each of the above two methods and two apparatus, the molded polymer may, for example, be an overmold, the substrate may be an electronic component, and the article may be a heat sink.
The foregoing and other features and advantages of the present invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawing.